Thanks again for the replies. Nothing like hearing the voice of experience.

I completely agree with the Prodigy as being the right choice. The cost savings of a few dollars wasn't my concern - I was more interested in knowing why THAT particular controller was so popular, and nobody ever said anything about another model or brand. The installation company I used to put in my OEM hitch happens to be a Tekonsha dealer, and the owner (my installer) was the one who asked the "Isn't a Prodigy kind of overkill for what you need to do?" question.

As it happens, my installer is also a real pro at what he does. I took the install instructions referenced above to him yesterday, and we discussed what he would do. Once he understood why the X5 is different than other cars, he said, "Actually, this install is more like it used to be, before cars were so complicated. It'll be a piece of cake." So, armed with the OEM pin part I had ordered (which looks surprisingly like the pin shown in the installation instructions already mentioned) he's confident he'll be able to do a fine job.

I'll report back once everything is done, and put together that definitive how-to I was looking for. Maybe the X5World moderators can offer it to those like me who were looking for a "final answer" about this. (I'm still amazed after this many X5 years that BMW hasn't stepped up with a suitable controller and installation instructions that tell folks exactly what to do.)

Thanks again for all the great info. You guys rock!

Dave